Heat the oven to 350°F (175°C). Lightly coat a 9×13-inch pan in oil or line it using parchment. Place the cake mix, softened cream cheese, eggs, and buttermilk in one bowl and beat until you have a smooth, uniform mixture.
Pour the batter into the prepared pan and level the surface. Bake for 30–35 minutes, or until a toothpick pushed into the center comes out clean.
Take the pan from the oven and let the cake sit for about 10 minutes. Use the handle of a wooden spoon or a skewer to poke holes across the surface, spacing them roughly an inch apart.
Blend the softened cream cheese and softened butter until smooth. Add powdered sugar, milk, almond extract, and vanilla. Mix until the texture is loose enough to pour; add a small splash of milk if needed.
Slowly pour the warm filling over the cake, aiming to let it run into the holes. Spread gently so it sinks in. Scatter the sliced almonds across the surface and press lightly so they stick.
Allow the cake to cool fully, and then place it in the refrigerator for at least an hour so the filling settles. Cut and serve chilled or at room temperature.